The present invention relates to a pickoff system for use with an inertial navigation assembly and particularly to a digitized pickoff system having a relaxation oscillator for use with an inertial sensor of an inertial navigation assembly.
The digitized pickoff system according to the present invention is intended to generate a digitized error signal from the pickoff elements of conventional inertial sensors, i.e., gyroscopes and accelerometers. The digital error may then be introduced to microcomputer circuitry wherein capture loop stabilization is effected.
The conventional inertial sensors are equipped with either variable inductance or variable capacitance error pickoffs, composed of two diametrically opposed elements. Displacement error of the sensor causes air gap variations which increase the reactance of one element and reduce that of its companion element.
The prior art pickoff system, by which pickoff error signal is generated, relies on connections of pickoff elements in series and exciting each end of the combination with a different polarity of balanced carrier signals referred to ground. An error signal, relative to ground, then appears at the junction of the pickoff elements. This signal is normally introduced to a carrier amplifier, then to a demodulator, excited by a carrier reference, whose output is the analog of displacement error in the sensor. The prior art pickoff system includes capture loops for these sensors, which require the inclusion of the above circuitry and the use of an analog to digital converter between the analog error and the digital processor input.
The primary disadvantage of the prior art pickoff system is the extent of circuitry required, i.e., a reference excitation source, an excitation transformer, a preamplifier, a demodulator and a standard analog to digital converter.
A second disadvantage of the prior art pickoff system is the sensitivity of digital output to offset of the demodulator and the analog to digital converter, compared to virtual insensitivity to amplifier offset voltage in the circuit in the invention.